Project Summary Maintaining epithelial homeostasis is crucial for organismal health since disruption of epithelial homeostasis promotes many human diseases, including cancers. Unnecessary, defective, or potentially harmful cells can be eliminated from epithelia by ?cell extrusion??a process to remove cells from epithelia without disrupting its barrier function. Previous studies have shown that cells can extrude from epithelia either apically or basally. Since the apical side of epithelia faces the outside or lumen, extrusion of cells toward the apical side generally leads to the shedding of extruded cells. In contrast, oncogenic transformation can make cells extrude toward basal side of the epithelia. When these basally extruded cells do not die off, they can spread to stroma and/or other tissues, which can be deleterious to the tissue and organism as a whole. Although basal cell extrusion is a crucial process for maintaining tissue homeostasis and initiating cancer metastasis, its cellular and molecular mechanisms are poorly understood. We have recently established a new Drosophila intestinal model of basal cell extrusion. We discovered that intestinal stem cells expressing oncogenic Ras (RasV12) extruded basally from intestinal epithelia in adult Drosophila. This model provides us a unique opportunity to uncover the fundamental mechanisms impinging on basal-cell extrusion using the state-of-the-art genomic and genetic tools available in Drosophila. With this model, we propose to address a few outstanding questions in the field: (1) we will define the cellular processes required for basal cell extrusion and determine their signaling networks; (2) we will scrutinize another long- standing question that epithelial-mesenchymal transition, a tumor-promoting process, is required for basal cell extrusion; and (3) we will elucidate the role of innate immune cells in the elimination of transformed cells via basal cell extrusion. Discovering the molecular basis underlying basal cell extrusion will help us to learn how to tweak the process to keep epithelia healthy and to prevent tumor metastasis.